core: Set custom RTC differential on game boot

src/core/core.cpp

@@ -30,6 +30,7 @@
 #include "core/hle/service/sm/sm.h"
 #include "core/loader/loader.h"
 #include "core/perf_stats.h"
+#include "core/settings.h"
 #include "core/telemetry_session.h"
 #include "frontend/applets/software_keyboard.h"
 #include "video_core/debug_utils/debug_utils.h"
@@ -94,6 +95,12 @@ struct System::Impl {
         CoreTiming::Init();
         kernel.Initialize();
 
+        const auto current_time = std::chrono::duration_cast<std::chrono::seconds>(
+                                      std::chrono::system_clock::now().time_since_epoch())
+                                      .count();
+        Settings::values.custom_rtc_differential =
+            Settings::values.custom_rtc.value_or(current_time) - current_time;
+
         // Create a default fs if one doesn't already exist.
         if (virtual_filesystem == nullptr)
             virtual_filesystem = std::make_shared<FileSys::RealVfsFilesystem>();